Since its premiere (and even before it), Joker has been stirring up controversy and questions. The film has received mixed reviews, with some claiming that Phoenix is on his way to another Oscar nomination and others saying that the whole film is bleak and too focused on stirring up controversy to actually be good. But controversy was indeed stirred, especially in regards to an important question that it never quite answers: Are Batman and the Joker brothers?
Where's the question coming from?
When Arthur confronts Penny about the letter, she tells him that she once worked for Thomas and the two had an affair, which led to Arthur. Since Thomas was a public figure, he couldn't risk having a child out of wedlock and took action to make it look like he wasn't the father.
When Arthur gets the chance to confront Thomas, he tells Arthur that the affair never happened and that Penny was obsessed with him. Arthur finds his mother's records at Arkham Asylum that state he was abandoned as a child and adopted by Penny, whose boyfriend caused severe psychological trauma to them both. It also claimed that Penny is a compulsive liar suffering from psychosis.
All of this information would point toward Thomas not being the father, except that Penny claims he falsified her records.
Even more so, the final scene of the film points to that, perhaps, none of it is true. It seems that the Joker has been locked in Arkham Asylum the whole time and, maybe, his whole story, is just one long figment of his imagination.
So are Batman and the Joker brothers?
If we are going off of the film alone, the answer would appear to be "maybe." Everything in the film is a "maybe." Maybe the story is truly his origin. Maybe it's a figment of his imagination. Maybe none of these people exist at all and are just in his head.
However, the Joker's potential father did weigh in on his opinion with The Hollywood Reporter.
Brett Cullen plays Thomas Wayne, Bruce Wayne's father and Arthur Fleck's maybe father. In an interview with The Hollywood Reporter, he was asked if he believed that Arthur was Thomas's son. Brett confirmed that he played his scenes with the idea that Thomas was indeed Arthur's father and he'd spent his life trying to discredit Penny's claims by putting her in mental institutions.
"The backstory was that Arthur’s mother had worked for Thomas in his home, and she was a beautiful woman who Thomas was attracted to and it led to a physical relationship," Cullen said. "Later in life, she’s in and out of mental institutions. And in my mind, Thomas Wayne put her there."
Whether Brett is correct or not is never confirmed in the film, but we just might see confirmation in a sequel, if it ever comes to happen.